(1) Field of the Invention
The present invention relates to seismic detection, for example to seismic detection carried out down a bore-hole to detect and measure seismic activity as represented by particle velocity or particle acceleration.
(2) Description of Related Art
It is already known to use, for such purposes, seismic detectors which have sensors oriented along three axes preferably at right angles to one another.
However, in such a detector, using three axial sensors, if one sensor (or the electronics associated with it) should fail, then the resulting two-component detector cannot give a representation of the three-dimensional movement which it is attempting to measure. Only a two-dimensional projection of this three-dimensional motion on to a plane can then be measured.
Also, the margin of error in such a three-axis detector is considerable since the xe2x80x98error inflation factorxe2x80x99 (i.e. the relationship between the error propagated from the measurement to the final estimate) is substantially 1 for each axis of a three-component system which means that for such a system the errors are compounded in the final estimates.
Moreover, there is no scope for cross-checking in such a three-axis detector. GB-A- 2 275 337 describes a seismic detector comprising a sonde which includes a configuration of four sensors (typically accelerometers or geophones) mounted in an equi-angular tetrahedral configuration with respect to one another to deal with the above problems. The four-sensor arrangement provides for some redundancy in the system such that the failure of one sensor still allows particle motion to be reconstructed in three dimensions (3D) and furthermore some form of error determination can be made, neither of which can be effected by the conventional three-sensor system. However, there is no disclosure of the processing required to realise these advantages, nor the processing required to extract the required seismic information from the configuration.
According to the present invention, there is provided, a method of using a seismic detector including four seismic sensors having axes which are in a substantially tetrahedral configuration, each of the sensors being in a respective signal channel, the method including one or more of the following steps:
a) combining outputs from the sensors to check that their polarities are correct;
b) testing to ascertain if one of the sensors is not working and, if so, using the outputs from the other three sensors;
c) if all four sensors are working, using their outputs to obtain an indication of motion in three dimensions on a least squares basis;
d) checking that the outputs from the sensors are coherent; and
e) checking the gains (or sensitivities) of the four channels.